Group III nitride compound semiconductors have direct transition and whose emission spectrum can be changed over a wide range from UV to red when used in a device such as a light-emitting device. Therefore Group III nitride compound semiconductor have been used in light-emitting devices such as a light-emitting diodes (LEDs) and laser diodes (LDs). In addition, since a Group III nitride compound semiconductor has a wide band gap, a device employing the semiconductor is considered to be operated reliably at high temperature, as compared with a device employing a semiconductor other than a Group III nitride compound semiconductor. Therefore, applying group III nitride compound semiconductors to electron devices including an FET, have been developed. Moreover, because arsenic (As) is not contained in Group III nitride compound semiconductors as a main constitution element, application of the semiconductors to the various semiconductor devices has been expected from the environmental viewpoint.
In general, when a metallic layer is merely formed on the surface of a compound semiconductor, ohmic contact between the metallic layer and the compound semiconductor fails to be obtained. Therefore, conventionally, the ohmic contact can be obtained by thermal treatment of the sample to diffuse the metal in the semiconductor. In the case of a p-type Group III nitride compound semiconductor, a resistivity of the p-type Group III nitride semiconductor is not reduce to the same level of that of n-type semiconductor, even the sample is taken heat treatment process or electron beam irradiation process. Therefore, the current does not spread in a lateral direction in the p-type layer, but flows just below electrade. Accordingly light is emitted merely from a portion directly beneath the electrode. To solve this problem, there has been proposed a current-diffusing electrode which is formed by laminating a nickel (Ni) layer (thickness: some hundreds Å) and a gold (Au) layer (thickness: some hundreds Å) and performing heat treatment thereafter, which exhibits light transmittance and ohmic characteristics (Japanese Patent Application Laid-Open (kokai) No. 6-314822). However, this electrode of two-layer structure including a nickel (Ni) layer and a gold (Au) layer has a contact resistivity as high as 2×10−3 Ωcm2 when the electrode contacts with a p-type Group III nitride compound semiconductor but the resistivity is still high. Therefore, a Group III nitride compound semiconductor device having this electrode still has a high operation voltage.